Shared slab awg circuits and systems

ABSTRACT

Planar AWG circuits and systems are disclosed that use air trench bends and shared MMI slabs to increase planar circuit compactness.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/271,541 entitled “ULTRA-COMPACT PLANAR AWG CIRCUITS ANDSYSTEMS” filed 4 Dec. 2007 for Gregory P. Nordin, Yongbin Lin, andSeunghyun Kim. The aforementioned application is incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates generally to lightwave circuits. Specifically, theinvention relates to compact planar AWG (arrayed waveguide grating)circuits and systems.

DESCRIPTION OF THE RELATED ART

Arrayed waveguide gratings typically include curved bends that providepathlength differences that enable interferometric multiplexing anddemultiplexing of lightwave signals. However, curved bends consumeconsiderable circuit area on a planar circuit substrate and typicallyrequire substantial layout changes in response to material changes.

SUMMARY OF THE INVENTION

The present invention has been developed in response to the presentstate of the art, and in particular, in response to the problems andneeds in the art that have not yet been fully solved by currentlyavailable planar lightwave circuits and systems. Accordingly, thepresent invention has been developed to provide a planar lightwavecircuit and system that overcome many or all of the above-discussedshortcomings in the art.

The present invention provides distinct advantages over the prior art.Reference throughout this specification to features, advantages, orsimilar language does not imply that all of the features and advantagesthat may be realized with the present invention should be or are in anysingle embodiment of the invention. Rather, language referring to thefeatures and advantages is understood to mean that a specific feature,advantage, or characteristic described in connection with an embodimentis included in at least one embodiment of the present invention. Thus,discussion of the features and advantages, and similar language,throughout this specification may, but do not necessarily, refer to thesame embodiment.

Furthermore, the described features, advantages, and characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. One skilled in the relevant art will recognize that theinvention may be practiced without one or more of the specific featuresor advantages of a particular embodiment. In other instances, additionalfeatures and advantages may be recognized in certain embodiments thatmay not be present in all embodiments of the invention.

These features and advantages will become more fully apparent from thefollowing description and appended claims, or may be learned by thepractice of the invention as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the invention will be readilyunderstood, a more particular description of the invention brieflydescribed above will be rendered by reference to specific embodimentsthat are illustrated in the appended drawings. Understanding that thesedrawings depict only typical embodiments of the invention and are nottherefore to be considered to be limiting of its scope, the inventionwill be described and explained with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIG. 1 is a top view planar circuit diagram depicting one example of anultracompact AWG (arrayed waveguide grating) circuit that is consistentwith at least one embodiment of the present invention;

FIG. 2 is a top view planar circuit diagram depicting another example ofan ultracompact AWG circuit that is consistent with at least oneembodiment of the present invention;

FIG. 3 is a perspective view planar circuit diagram depicting oneexample of a reflective bend that is consistent with at least oneembodiment of the present invention;

FIG. 4 is a perspective view planar circuit diagram depicting certainaspects of a ribbed waveguide that is consistent with at least oneembodiment of the present invention;

FIG. 5 is a perspective view planar circuit diagram depicting anotherexample of a reflective bend that is consistent with at least oneembodiment of the present invention;

FIG. 6 is a top view circuit diagram comparing the relative size of oneAWG circuit of the present invention with a prior art AWG circuit;

FIG. 7 is a top view planar circuit diagram depicting one example of alayout methodology that is consistent with at least one embodiment ofthe present invention;

FIG. 8 is a top view planar circuit diagram depicting another example ofa layout methodology that is consistent with at least one embodiment ofthe present invention;

FIG. 9 is a top view planar circuit diagram depicting yet anotherexample of a layout methodology that is consistent with at least oneembodiment of the present invention; and

FIG. 10 is a top view planar circuit diagram depicting certain aspectsof a shared slab that is consistent with at least one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention. Thus,appearances of the phrases “in one embodiment,” “in an embodiment,” andsimilar language throughout this specification may, but do notnecessarily, all refer to the same embodiment.

Furthermore, the described features, structures, or characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. In the following description, numerous specific details areprovided, such as examples of lightwave circuits and fabricationtechniques, etc., to provide a thorough understanding of embodiments ofthe invention. One skilled in the relevant art will recognize, however,that the invention may be practiced without one or more of the specificdetails, or with other methods, components, materials, and so forth. Inother instances, well-known structures, materials, or operations are notshown or described in detail to avoid obscuring aspects of theinvention.

FIGS. 1 and 2 are top view planar circuit diagrams depicting twoexamples of an ultracompact AWG (arrayed waveguide grating) circuit 100.As depicted, each AWG circuit 100 includes one or more input waveguides110, one or more output waveguides 120, an input MMI (multi-modeinterference) slab 130, an output MMI (multi-mode interference) slab140, and a set of grating waveguides 150.

The input waveguides 110 are photonically connected to the input MMIslab 130 via a corresponding set of input ports 132. Similarly, theoutput MMI slab 140 is photonicly connected to the output waveguides 120via a corresponding set of output ports 142. The input waveguides 110and output waveguides 120 may bear one or more optical channels. One ofskill in the art will appreciate that the number of input waveguides 110and output waveguides 120 and the number of channels each waveguidebears may be dependent on whether the AWG circuit 100 functions as amultiplexing device or a demultiplexing device.

The grating waveguides 150 photonically connect the input MMI slab 130to the output MMI slab 140. To accomplish the multiplexing ordemultiplexing functionality of the AWG circuit 100, each gratingwaveguide has a unique path length and therefore a unique phase delaybetween the input MMI slab 130 and the output MMI slab 140. Theinteraction of the unique phase delays and the interferometric effectsof the MMI slabs 130 and 140 result in the desired multiplexing ordemultiplexing of optical signals.

The waveguides 110, 120, and 150 have a low index contrast relative tothe (planar) cladding material in which they are formed. Providing bendsin low index contrast waveguides has typically required a very gradualcurvature in order to retain optical signals within each waveguide. Aswill be noted in greater detail in FIG. 6, the use of a very gradualcurvature has resulted in AWG devices that consume large amounts ofplanar circuit area.

The planar compactness of the AWG circuit 100 in general, and thegrating waveguides 150 in particular, is achieved by the use ofreflective bends 160 that interconnect a number of segments 170 withineach grating waveguide 150. Similarly, the input waveguides 110 andoutput waveguides 120 may be substantially more compact thancorresponding prior art solutions by using one or more reflective bends160 to interconnect the segments 170 within each waveguide 110 and 120.The segments 170 may be substantially straight segments that propagateoptical signals with little or no loss. As will be described in greaterdetail for certain embodiments presented in FIGS. 3 and 5, thereflective bends 160 redirect optical signals from one waveguide segmentto another and enable effective lightwave propagation with refractionindex contrasts of less than 5 percent.

In the depicted embodiments, the input MMI slab 130 and the output MMIslab 140 are adjacent to each other and substantially parallel with eachother. The adjacency of the input MMI slabs 130 and output MMI slabs 140substantially reduces the rectangular surface area required to build theAWG circuit 100 and facilitates efficient integration with relatedplanar circuits as is illustrated in FIGS. 7 and 8.

The number of waveguide segments 170 and reflective bends 160 that arerequired to achieve a desired bending angle and/or slab adjacency may bedependent on the refractive index of the waveguide material. The AWGcircuit 100 a depicted in FIG. 1 uses four reflective bends 160 a ofapproximately 45 degrees to achieve a desired bending angle of 180degrees within each grating waveguide 150 a. Similarly, the depictedinput waveguides 110 a and output waveguides 120 a use two reflectivebends of approximately 45 degrees to achieve a 90 degree bending angle.In one embodiment, the waveguides and reflective bends of FIG. 1 areconstructed of a low index material in the manner depicted in FIG. 3.

The AWG circuit 100 b depicted in FIG. 2 uses two reflective bends 160 bof approximately 90 degrees to achieve a 180 degree bending angle withinthe grating waveguides 150 b while the depicted input waveguides 110 band output waveguides 120 b use a single reflective bend ofapproximately 90 degrees. In one embodiment, the waveguides andreflective bends of FIG. 2 are constructed of a high index material inthe manner depicted in FIGS. 4 and 5.

FIG. 3 is a perspective view planar circuit diagram depicting oneexample of a reflective bend 300 that is consistent with at least oneembodiment of the present invention. As depicted, the reflective bend300 includes a planar medium or cladding 310, an input segment 320, anoutput segment 330, an air trench 340, and a TIR surface 350. Thereflective bend 300 is one example of the reflective bend 160 a depictedin FIG. 1.

The input segment 320 and the output segment 330 may have substantiallyidentical indices of refraction. In most embodiments, the input segment320 and output segment 330 are simultaneously formed of the samematerial and embedded in the planar medium 310. The planar medium orcladding 310 may have an index of refraction that is slightly differentfrom the input segment 320 and the output segment 330 and thus provide alightwave waveguide of low index contrast. One of skill in the art willappreciate that a wide variety of microfabrication techniques may beused to embed the segments 320 and 330 within the planar medium 310.

In order to bear lightwave signals with little or no loss, the inputsegment 320 and output segment 330 may be substantially straightsegments. Generally, the input segment 320 and the output segment 330are a portion of two interconnecting segments 170 within a waveguide.

An air trench 340 may be formed in the reflective bend 300 to provide aTIR (total internal reflective) surface 350. In the depicted embodiment,the TIR surface 350 is achieved with a single air trench 340. The TIRsurface 350 may include a portion of the input segment 320 and theoutput segment 330. Generally, the output segment 330 is disposed at abending angle relative to the input segment 320. The bending angle maybe selected to ensure that lightwave signals experience total internalreflection at the TIR surface 350.

In the depicted reflective bend 300, the refractive index of thecladding material 310 and the segments 320 and 330 is generally lessthan 2.5. In the examples depicted herein, the bending angles for thereflective bends are selected to ensure that at least 90 percent oflightwave energy provided by the input segment 320 is reflected by theTIR surface to the output segment 330. For segments of a relatively lowindex of refraction such as segments formed in silica, a bending angleof less than 70 degrees may be required to attain total internalreflection and propagate at least 90 percent of the lightwave energyprovided to the input segment 320.

FIG. 4 is a perspective view planar circuit diagram depicting certainaspects of a ribbed waveguide 400 that is consistent with at least oneembodiment of the present invention. As depicted, the ribbed waveguide400 includes a substrate 410, a rib 420, a waveguide region 430, andcladding regions 440 a and 440 b. As is known to those of skill in theart, forming a rib 420 in certain high index materials such as siliconmay provide a region having a slightly higher refractive index that maybe used as a waveguide region 430 between the cladding regions 440.

FIG. 5 is a perspective view planar circuit diagram depicting anotherexample of a reflective bend 500 that is consistent with at least oneembodiment of the present invention. As depicted, the reflective bend500 includes an input segment 510 under an input rib 520, an outputsegment 530 under an output rib 540, an input cladding region 550, anoutput cladding region 560, an air trench 570, and a reflective surface580. The reflective bend 500 is one example of the reflective bend 160 bdepicted in FIG. 2.

Formation of the input rib 520 and the output rib 540 may providewaveguide regions below each rib having a slightly increased index ofrefraction resulting respectively in the input (waveguide) segment 510and the output (waveguide) segment 530 as well as the input claddingregion 550 and the output cladding region 560. In most embodiments, theinput rib 520 and output rib 540 are simultaneously formed of the samematerial. In certain embodiments, the reflective bend 500 is formed of ahigh index material having a refractive index of greater than 2.0 suchas silicon.

In order to bear lightwave signals with little or no loss, the inputsegment 510 and output segment 530 may be substantially straight.Generally, the input segment 510 and the output segment 530 are aportion of two interconnecting segments 170 within a waveguide. An airtrench 570 may be formed in the reflective bend 580 to provide a TIR(total internal reflective) surface 580. In the depicted embodiment, asingle air trench 570 is used to provide the TIR surface 580.

The depicted TIR surface 580 includes a portion of the input segment 510and the output segment 530. The output segment 530 may be disposed at abending angle relative to the input segment 510. The bending angle maybe selected to ensure that lightwave signals experience total internalreflection at the TIR surface 580. Generally, the bending angle isselected to ensure that at least 80 percent of lightwave energy providedby the input segment 510 is reflected by the TIR surface 580 to theoutput segment 530. For segments of a relatively high index ofrefraction such as segments formed in silicon, a bending angle of lessthan 120 degrees may be required to attain total internal reflection andpropagate at least 80 percent of the lightwave energy provided to theinput segment 510.

FIG. 6 is a top view circuit diagram comparing the relative size of oneAWG circuit 610 of the present invention with a prior art AWG circuit620. As depicted, the AWG circuit 610 occupies a planar rectangular areaof approximately 12.6 square millimeters while the prior art AWG circuitoccupies a planar rectangular area of approximately 259 squaremillimeters.

The use of reflective bends and the resulting compactness may increasetemperature stability. In the depicted embodiment, a temperature shiftless 0.012 nm/C has been demonstrated for the AWG circuit 610 versus0.059 nm/C for a prior art AWG circuit of similar capability. Anotheradvantage is a large degree of material independence due to the use oftotal internal reflection (TIR) surfaces. In certain embodiments, otheradvantages include compatibility with batch micro-fabrication techniquesand reduced birefringence.

FIGS. 7 and 8 are top view planar circuit diagrams depicting twoexamples of layout methodologies 700 that are consistent with at leastone embodiment of the present invention. As depicted, the layoutmethodologies 700 include one or more AWG circuits 100 interconnectedwith related planar circuits such as related planar circuits 720 a, 720b, and 720 c. The related planar circuits 720 may be selected from, orinclude, a variety of circuits such as optical amplifiers, electronicamplifiers, add/drop circuits, sensors, A/D converters, D/A converters,control units, and the like, that enable highly integrated lightwavesystems such as optical switches, add/drop multiplexers, and the like.

The depicted methodologies facilitate alignment of input and outputwaveguides and increased system compactness. The layout methodology 700b shown in FIG. 8 aligns all input and output waveguides on the same endof the AWG circuits 100 while the layout methodology 700 a shown in FIG.7 alternates ends for the AWG circuits 100 so that the optical signalsprocessed by the related circuits 720 may flow from one end of thecircuit to the other end.

FIG. 9 is a top view planar circuit diagram depicting one example of alayout methodology 900 that is consistent with at least one embodimentof the present invention. As depicted, the layout methodology 900includes multiple port sets 910, a shared MMI slab 920, and gratingwaveguide sets 930. The layout methodology 900 improves the compactnessof AWG circuits and systems.

The port sets 910 provide input ports 910 a and output ports 910 d thatenable connections to and from other optical or electro-opticalcircuits. The input ports 910 a may provide a beam 940 such as aGaussian beam or the like to the shared MMI slab 920 which expands as itcrosses the MMI slab 920. Due to the diameter of the beam 940 at theoutput ports 910 b, the MMI slab 920 photonically couples the inputports 910 a to corresponding output ports 910 b without providingsubstantial photonic coupling to other output ports 910 b.

The output ports 910 b connect the MMI slab to the corresponding gratingwaveguide set 930. Each grating waveguide of the corresponding gratingwaveguide set 930 provides a unique optical path length from, and backto, the shared MMI slab 920. The input ports 910 c connect thecorresponding grating waveguide set 930 back to the MMI slab 920. TheMMI slab 920 and the placement of the ports function to photonicallycouple the beam energy returned by the input ports 910 c to the outputports 910 d.

One of skill in the art will appreciate that the overall path lengthdifference between the each of input ports 910 a and each of the outputports 910 d within a port set 910 enables multiplexing or demultiplexingof various channels via multi-mode interference. One of skill in the artwill also appreciate the compactness that is attainable with the layoutmethodology 900. In one embodiment, 8 AWG circuits with 8 inputs and 8outputs in each AWG are contained within a planar area of 100 squaremillimeters.

FIG. 10 is a top view planar circuit diagram depicting certain aspectsof a shared slab that is consistent with at least one embodiment of thepresent invention. As depicted, the shared slab 1000 includes a slab1010, a variety of input ports 1020, and a variety of output ports 1030.The shared slab 1000 may be used to form an AWG system such as thesystem 900 shown in FIG. 9.

The depicted slab 1010 is rectangular in shape. However, one of skill inthe art will appreciate that the slab 1010 may assume a variety ofshapes that provide the desired spatial relationship between the inputports 1020 and the output ports 1030.

The input ports 1020 are arrange in ports sets 1040 while the outputports are arranged in port sets 1050. Each input port 1020 within a portset 1040 may provide a beam 1025 that illuminates each output port 1030within a corresponding (i.e. photonically coupled) port set 1050 whichis indicated in the reference numbers of FIG. 9 with a common lettersuffix.

The input ports 1020 may be angled to provide better illumination to thecorresponding port set 1050. Although some energy from a beam 1025 mayimpinge upon other output ports, the output ports within non-coupledport sets 1050 are not within the beam diameter of the beams 1025provided by a port set 1040. With a Gaussian beam, for example, the beamdiameter may be defined by a conical asymptotic surface that correspondsto a 1/e reduction in the strength of the electric field relative to theelectric field strength at the axis of the beam.

The preceding description has been presented only to illustrate anddescribe a variety of example embodiments and implementations withreference to the accompanying drawings. Despite the details providedherein, it will be evident to one of skill in the art that variousmodifications and changes may be made, and additional implementationsmay be implemented, without departing from the scope of the invention asset forth in the claims that follow. The above description andaccompanying drawings are accordingly to be regarded in an illustrativerather than a restrictive sense.

1. A planar lightwave circuit comprising: an MMI (multi-modeinterference) slab comprising a first set of input ports comprising atleast two ports and a second set of input ports comprising at least twoports; the MMI slab further comprising a first set of output portscomprising at least two ports and a second set of output portscomprising at least two ports; the first set of input ports positionedto photonically couple each of the first set of input ports to each ofthe first set of output ports without substantial photonic couplingbetween each of the first set of input ports and each of the second setof output ports; the second set of input ports positioned tophotonically couple each of the second set of input ports to each of thesecond set of output ports without substantial photonic coupling betweeneach of the second set of input ports and each of the first set ofoutput ports; and a plurality of grating waveguides configured tophotonically connect the first set of output ports to the second set ofinput ports.
 2. The invention of claim 1, wherein the first set of inputports and the first set of output ports are substantially orthogonal tothe second set of input ports and the second set of output ports.
 3. Theinvention of claim 1, wherein the plurality of grating waveguides areembedded in a cladding material, the cladding material having arefractive index of less than 2.5 and the plurality of gratingwaveguides having an index contrast of less than 5 percent relative tothe cladding material.
 4. The invention of claim 1, herein each gratingwaveguide of the plurality of grating waveguides comprises a pluralityof straight segments interconnected with a plurality of reflectivebends, each reflective bend comprising: an input segment, an outputsegment disposed at a bending angle of less than 70 degrees relative tothe input segment, and a single air trench forming a TIR (total internalreflective) surface on a portion of the input segment and the outputsegment, the TIR surface configured to reflect at least 80 percent oflightwave energy provided by the input segment to the output segment. 5.The invention of claim 1, further comprising a related planar circuitthat is connected with and adjacent to the MMI slab.
 6. The invention ofclaim 5, wherein the related planar circuit is an add/drop circuit. 7.The invention of claim 5, wherein the related planar circuit comprisesat least one circuit selected from the group consisting of an opticalamplifier, an electronic amplifier, an add/drop circuit, a sensor, anA/D converter, a D/A converter, a variable attenuator, a tap, awavelength converter, a phase shifter, a filter, and a control unit. 8.The invention of claim 1, wherein the MMI slab and grating waveguidescollectively fit within a planar rectangular area of less than 15 squaremillimeters.
 9. The invention of claim 1, wherein a grating waveguide ofthe plurality of grating waveguides is a ribbed waveguide formed of amaterial having a refractive index of greater than 2.0, the ribbedwaveguide configured to provide an index contrast of less than 5percent.
 10. The invention of claim 9, wherein the ribbed waveguidecomprises a plurality of straight segments interconnected with aplurality of reflective bends, each reflective bend comprising: an inputsegment, an output segment disposed at a bending angle of less than 120degrees relative to the input segment, and a single air trench forming aTIR (total internal reflective) surface on a portion of the inputsegment and the output segment, the TIR surface configured to reflect atleast 80 percent of lightwave energy provided by the input segment tothe output segment.